Froth floatation is based upon

Sulphide ores of metals are usually concentrated by froth floatation process. Which one of the following sulphide ores offers an exception and is concentrated by chemical leaching?

Match the method of concentration of the ore in column $I$ with the ore in column $II$ and select the correct alternate :

$I$	$II$
$X$. Magnetic separation	$a$. $Ag_2S$
$Y$. Froth floatation	$b$. $FeCr_2O_4$
$Z$. Gravity separation	$c$. $Al_2(SiO_3)_3$

Explain the froth flotation process.

Which step is involved in the extraction of $Pb$ from galena $(PbS)$?

Which one of the following is not a method of concentration of metals?